In mobile communication, ARQ (Automatic Repeat Request) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to “base station”) to radio communication mobile station apparatuses (hereinafter abbreviated to “mobile stations”). That is, mobile stations feed back response signals representing error detection results of downlink data, to the base station. Mobile stations perform a CRC (Cyclic Redundancy Check) of downlink data, and, if CRC=OK (no error), feed back an ACK (ACKnowledgement), or, if CRC=NG (error present), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station. These response signals are transmitted to the base station using uplink control channels such as a PUCCH (Physical Uplink Control CHannel) and uplink L1/L2 CCH (L1/L2 Control CHannel).
Also, as shown in FIG. 1, studies are underway to perform code-multiplexing by spreading a plurality of response signals from a plurality of mobile stations using ZC (Zadoff-Chu) sequences and Walsh sequences (see Non-Patent Document 1). In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4. As shown in FIG. 1, first, an ACK or NACK response signal from a mobile station is subject to first spreading in one symbol by a ZC sequence (having a sequence length of 12) on the frequency domain. Next, the response signal subjected to first spreading is subject to an IFFT (Inverse Fast Fourier Transform) in association with W0 to W3. The response signal spread on the frequency domain by a ZC sequence having a sequence length of 12 is transformed to a ZC sequence having a sequence length of 12 on the time domain by this IFFT. Then, the signal subjected to the IFFT is subject to second spreading using a Walsh sequence (having a sequence length of 4). That is, one response signal is allocated to four symbols S0 to S3 individually. Similarly, response signals of other mobile stations are spread using ZC sequences and Walsh sequences. Here, different mobile stations use ZC sequences of different cyclic shift values on the time domain, or different Walsh sequences. In this case, the sequence length of ZC sequence on the time domain is 12, so that it is possible to use twelve ZC sequences of the cyclic shift values “0” to “11,” generated from the same ZC sequence. Also, the sequence length of Walsh sequence is 4, so that it is possible to use four different Walsh sequences. Therefore, in an ideal communication environment, it is possible to code-multiplex maximum 48 (12×4) response signals from mobile stations.
Here, there is no cross-correlation between ZC sequences of different cyclic shift values generated from the same ZC sequence. Therefore, in an ideal communication environment, a plurality of response signals subjected to spreading and code-multiplexing by ZC sequences of different cyclic shift values (0 to 11), can be demultiplexed on the time domain without inter-code interference, by correlation processing in the base station.
However, due to the influence of, for example, transmission timing difference in mobile stations, multipath delayed waves and frequency offsets, a plurality of response signals from a plurality of mobile stations do not always arrive at a base station at the same time. For example, if the transmission timing of a response signal spread by a ZC sequence of the cyclic shift value “0” is delayed from the correct transmission timing, the correlation peak of the ZC sequence of the cyclic shift value “0” may appear in the detection window for the ZC sequence of the cyclic shift value “1.” Further, if a response signal spread by the ZC sequence of the cyclic shift value “0” has a delay wave, interference leakage due to the delayed wave may appear in the detection window for the ZC sequence of the cyclic shift value “1.” That is, in these cases, the ZC sequence of the cyclic shift value “1” is interfered by the ZC sequence of the cyclic shift value “0.” Therefore, in these cases, the separation performance degrades in a response signal spread by the ZC sequence of the cyclic shift value “0” and a response signal spread by the ZC sequence of the cyclic shift value “1.” That is, if ZC sequences of adjacent cyclic shift values are used, the separation performance of response signals may degrade.
Therefore, up till now, if a plurality of response signals are code-multiplexed by spreading of ZC sequences, a cyclic shift interval is provided between the ZC sequences, in order to suppress inter-code interference between the ZC sequences. For example, when the cyclic shift interval between ZC sequences is 2, studies are underway to use only six ZC sequences of the cyclic shift values “0,” “2” “4” “6,” “8” and “10” in first spreading of response signals, amongst twelve ZC sequences of the cyclic shift values “0” to “11.” Therefore, if Walsh sequences having a sequence length of 4 are used in second spreading of response signals, it is possible to code-multiplex maximum 24 (6×4) response signals from mobile stations.
Also, the base station transmits control information for reporting resource allocation results of downlink data, to the mobile stations. This control information is transmitted to the mobile stations using downlink control channels such as PDCCH (Physical Downlink Control CHannel), downlink L1/L2 CCH (L1/L2 Control CHannel) and DL grant (DownLink grant) provided on a per mobile station basis. Each PDCCH occupies one or a plurality of CCE's (Control Channel Elements). If one PDCCH occupies a plurality of CCE's, the plurality of CCE's occupied by the PDCCH are consecutive. Based on the number of CCE's required to notify control information, the base station allocates an arbitrary PDCCH among the plurality of PDCCH's to each mobile station, maps control information on the physical resources associated with the CCE's occupied by the allocated PDCCH's, and performs transmission.
Also, to use downlink communication resources efficiently without signaling to report PUCCH's used for transmitting response signals from the base station to the mobile stations, studies are underway to associate CCE's with PUCCH's on a one-to-one basis. According to this association, each mobile station can decide the PUCCH to use to transmit a response signal from that mobile station, from the CCE's associated with the physical resources on which control information for that mobile station is mapped. That is, each mobile station maps a response signal from that mobile station on physical resources, based on the CCE's associated with physical resources on which control information for that mobile station is mapped.
Here, the number of CCE's occupied by PDCCH varies according to the MCS (Modulation and Coding Scheme) of the PDCCH. If a mobile station is located far from a base station and has lower received signal quality, the base station reduces the modulation level or coding rate of PDCCH while increasing the number of CCE's. By contrast, if the mobile station is located near the base station and has higher received signal quality, the base station increases the modulation level or coding rate of PDCCH while decreasing the number of CCE's. That is, a PDCCH of a lower MCS level occupies a larger number of CCE's, and a PDCCH of a higher MCS level occupies a smaller number of CCE's. In other words, a mobile station to which a PDCCH of a low MCS level is allocated provides a large number of CCE's, and a mobile station to which a PDCCH of a high MCS level is allocated provides a small number of CCE's. For example, when the coding rate of a PDCCH is one of ⅔, ⅓ and ⅙, and a PDCCH of the coding rate ⅔ occupies one CCE, an PDCCH of the coding rate ⅓ occupies two CCE's, and a PDCCH of the coding rate ⅙ occupies four CCE's.
Also, as described above, studies are underway to allow a single mobile station to transmit a response signal using only the PUCCH associated with the CCE of the minimum number among a plurality of CCE's in the case of allocating these CCE's to the mobile station (see Non-Patent Document 2).    Non-Patent Document 1: Multiplexing capability of CQIs and ACK/NACKs form different UEs (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-072315.zip)    Non-Patent Document 2: R1-072348, LG Electronics, “Allocation of UL ACK/NACK index”, 3GPP TSG-RAN WG1 Meeting #49, Kobe, Japan, May 7-11, 2007